1. Field of the Invention
This invention relates to a gas turbine combustor and a method for operating such a combustor so that the exhaust products flowed to a turbine will be relatively free of carbon particles. More specifically, this invention provides a combustor for a stored energy subsystem using aviation fuel and compressed air for burning a fuel rich mixture to yield a controlled, high temperature, high pressure gas whereby apparatus and method are provided for preventing the reaction from reaching completion.
2. Description of Related Art
Generally, an aircraft has one or more primary engines which provide thrust for the aircraft, as well as pressurized bleed air for the environmental control systems. The primary engine also provides power to drive electric generators and hydraulic pumps, both of which are necessary for powering instruments and flight control systems. In addition, many aircraft also have an auxiliary power engine to provide electric and hydraulic power, as well as bleed air to the aircraft when the primary engines are not operating, for example when the aircraft is on the ground. The auxiliary power engine may also provide power to start the primary engines either on the ground or in flight. Both the primary engine and the auxiliary power engine operate on aviation fuel from the aircraft's main fuel tanks, mixed with air drawn from the atmosphere as the combustion components. For maximum fuel efficiency, these engines operate in an air rich, fuel lean mode. In many instances, starting the auxiliary power engine requires an external power source such as a ground based start cart, a pressurized air tank, or an emergency power system. Since the auxiliary power engine is primarily designed to operate on the ground where the air is relatively dense, the auxiliary power engine may be incapable of operating at higher elevations, for example above 55,000 feet. It is therefore evident that in many applications the auxiliary power engine would not be able to restart a failed primary engine above 55,000 feet, and in this event there would be no electrical or hydraulic power available. Also, since both the primary engine and the auxiliary power engine operate on fuel from the main fuel tanks, if this fuel supply is depleted there will be no source of power for the electrical and hydraulic power systems to allow the pilot to control and land the aircraft.
It is therefore desirable to have on an aircraft an emergency power system capable of operating independent of external conditions which can provide emergency electrical and hydraulic power to the flight control systems and may be used to restart the auxiliary or primary engines. These are the minimum requirements of the emergency power system. Since they are only operated in the event of an emergency, these systems remain stored and inactive for long periods of time, but are required to start instantly and provide continuous power output for a prespecified duty cycle. Ideally, such an emergency power system would be compact, lightweight, highly reliable, easily maintained, require no special handling of materials or fuels, while providing a combustion process which is controllable and which produces a clean, combustion gas. Presently, emergency power units primarily rely on liquid hydrazine based fuels sprayed into a catalyst bed to generate a pressurized gas. These units are in use on several aircraft and combine high performance with low weight.
However, liquid hydrazine is highly corrosive and toxic, thereby requiring special handling procedures and design considerations. The catalyst material is expensive, and when the catalyst is depleted it must be replaced. Further, the combustion gas which is produced is toxic and therefore limits ground testing of the emergency power unit.
To overcome such problems emergency power systems were designed to operate on a fuel rich mixture of aviation fuel and air which optimizes the advantages of such a system to yield an emergency power system with the improved characteristics of relying on an energy source which is readily available, non-toxic, and clean burning, packaged in a compact, lightweight, highly reliable, and easily maintained emergency power unit.
Such an emergency power system is disclosed in U.S. Pat. No. 4,777,793 entitled "Emergency power unit" and its divisional U.S. Pat. Nos. 4,934,136 entitled "Method of Operating an Emergency Power Unit" and 4,898,000 entitled "Combustor for an Emergency Power Unit" assigned to the present assignee.
A problem with using carbon based jet fuels such as contemplated for the present invention is that in a fuel rich environment formation of solid particles occur, generally comprised of carbon, suspended in the combustion product gas flow. These particles pose a significant problem effecting the gas turbine's operation, durability, and reliability because it can cause erosion of gas nozzles and turbine blades as well as clog gas passages downstream of the combustor.